We continued our collaboration with Dr. R. Ghirlando of NIH, who determined weight average molecular weight (Mw) values of various hydroxyethyl starch (HES) volume expander products by low angle laser light scattering; scattering data at 514 nm are measured as a function of concentration and angle and are extrapolated to zero concentration and angle by means of a Zimm plot to obtain Mw. We need these data in order to assess the ability of manufacturers to determine accurate molecular weight values for HES products by HPLC-SEC. Until now only a single manufacturer among many has demonstrated the ability to do this; even the innovator is unable to determine accurate molecular weights. During the past year, Dr. Ghirlando has measured Mw of lots of Pentastarch from the innovator as well as lots of Hetastarch from a second generic manufacturer. The dn/dc values for Hetastarch and Pentastarch in 150 mM NaCl and in water have been corrected for moisture and NaCl content of the powders. No difference was found between values for Hetastarch and Pentastarch and no difference between values for measurements in 150 mM NaCl and in water. (dn/dc) lambda=546 for HES (Hetastarch or Pentastarch) in 150 mM NaCl or in water is 0.1451 mL/g. With regard to the proposed USP Monographs for Dextran 70 and Dextran 40, we composed a response to the USP about comments of a drug substance manufacturer concerning number average molecular weight (Mn) requirements proposed by the Agency. In our response, we utilized Mn data for Dextran 70 and Dextran 40 determined at CBER by membrane osmometry in conjunction with published Mn data for other dextrans with substantially different Mn values in plots of oncotic pressure (pi) vs. Mn. The manufacturer contends that Mn is relatively unimportant with regards to volume expansion and oncotic pressure but rather that the concentration and water binding capacity are the important factors. A plot of pi (at 25 or 40 mg/mL of dextran) vs. Mn shows an almost linear dependence of pi on Mn from 25,000 to ca. 50,000, which includes Dextran 40 (Mn approx. 24,000) and Dextran 70 (Mn approx. 40,000). In vivo, the effective Mn is greater than the actual Mn because the vascular bed is permeable to low molecular weight species, but effective Mn's for Dextran 70 and Dextran 40 are within the linear range. Thus, under physiological conditions both products develop oncotic pressures in response to the in vitro Mn values, which with Mw values, also permit an estimate of polydispersity.